As the multilayer printed circuit board, there is a build-up multilayer printed circuit board in which a conductor circuit and a resin insulating layer are alternately built-up and an inner circuit layer and an outer circuit layer are electrically connected to each other through viaholes or the like. This build-up multilayer printed circuit board is generally manufactured by a method wherein a layer of the conductor circuit is formed in a position closest to a substrate by a subtractive process and then a plurality of conductor circuit layers are laminated on the conductor circuit layer by an additive process.
The subtractive process is a method of forming the conductor circuit by subjecting a surface of a copper lined laminate to an etching treatment and can cheaply form the conductor circuit layer having a high reliability.
The additive process is a method wherein an adhesive for electroless plating is applied onto a substrate such as glass epoxy or the like to form a resin insulating layer and then the resin insulating layer is roughened and a plating resist is formed on the roughened surface and then a metal for conductor circuit is applied thereto by electroless plating. According to this method, the conductor circuit is formed on the roughened resin insulating layer through the plating or the like, so that the excellent adhesion property therebetween can be ensured and hence there can be manufactured a printed circuit board in which the conductor circuit hardly peels from the resin insulating layer.
In the multilayer printed circuit board, when the innermost conductor circuit is formed by the subtractive process, concave portion is caused between the conductor circuits formed by the etching treatment. Therefore, when the resin insulating layer is formed on the wiring substrate having the concave portion between the conductor circuits, as shown in FIG. 4(a), the surface of the resin insulating layer becomes uneven and hence there is caused a problem that the viahole shape formed in the resin insulating layer and junction pad are deformed to cause poor mounting.
Concretely, the resin insulating layer formed in a large area portion of the conductor circuit becomes thicker as compared with the resin insulating layer formed in a small area portion of the conductor circuit. As a result, the exposure and development conditions of the viahole vary in accordance with the thickness of the resin insulating layer, so that
1. When the exposure and development conditions are matched with the thin portion of the resin insulating layer, a developing residue is created in the viahole formed in the thick portion of the resin insulating layer to close the opening of the viahole and hence the connection to a wiring layer arranged on the upper layer becomes incomplete (see FIG. 4(b)). PA0 2. When the exposure and development conditions are matched with the thick portion of the resin insulating layer, the insulating material is peeled from the vicinity of the viahole formed in the thin portion of the resin insulating layer, which results in the poor insulation (see FIG. 4(c)). PA0 (1) A multilayer printed circuit board comprising an upper conductor circuit layer, a lower conductor circuit layer and a resin insulating layer electrically insulating both the conductor circuit layers, characterized in that said resin insulating layer is comprised of a composite layer consisting of an insulating layer made from a heat-resistant resin hardly soluble in acid or an oxidizing agent as a lower layer and an adhesive layer for electroless plating made from a heat-resistant resin as an upper layer. PA0 (2) In the invention described in the item (1), the insulating layer is preferable to contain a thermoplastic resin. PA0 (3) In the invention described in the item (1), the insulating layer is preferable to be a heat-resistant resin containing an organic resin filler hardly soluble in the acid or oxidizing agent. PA0 (4) In the invention described in the item (1), the adhesive layer is preferable to be an adhesive formed by dispersing cured particles of a heat-resistant resin soluble in the acid or oxidizing agent into a matrix of a heat-resistant resin hardly soluble in the acid or oxidizing agent. PA0 (5) In the invention described in the item (1), it is preferable that the adhesive layer has a thickness of 10-50 .mu.m and the insulating layer has a thickness of 10-100 .mu.m. PA0 (6) In the invention described in the item (3), it is favorable that the organic resin filler is particles of a heat-resistant resin hardly soluble in the acid or oxidizing agent and has a maximum particle size corresponding to not more than 8/10 of the thickness of the insulating layer and an average particle size corresponding to not more than 1/2 of the thickness of the insulating layer. PA0 (7) In the invention described in the item (3), it is favorable that a filling amount of the organic resin filler is 10-100 parts by volume per 100 parts by volume of the heat-resistant resin. PA0 (8) In the invention described in the item (3), it is favorable that the organic resin filler is made from at least one resin selected from the group consisting of epoxy resin, benzoguanamine resin, styreae resin, divinylbenzene resin and polyimide resin. PA0 (9) It is desirable that cured particles of a heat-resistant resin soluble in the acid or oxidizing agent and having an average particle size of 0.1-2.0 .mu.m are dispersed into the insulating layer as the lower layer. PA0 (10) A multilayer printed circuit board comprising an upper conductor circuit layer, a lower conductor circuit layer and a resin insulating layer electrically insulating both the conductor circuit layers, characterized in that said lower conductor circuit layer is formed by etching a substrate provided with a conductor layer to remove a conductor useless portion therefrom, and a resin is filled in a concave portion formed by removing the conductor useless portion so that the surface of the resin is same as the surface of the lower conductor circuit layer. PA0 (11) In the invention described in the item (10), it is preferable that the resin filled in the concave portion formed by removing the conductor useless portion is a cured product of non-solvent resin. PA0 (12) In the invention described in the item (10), it is preferable that the resin filled in the concave portion formed by removing the conductor useless portion contains inorganic particles. PA0 (13) A multilayer printed circuit board comprising an upper conductor circuit layer, a lower conductor circuit layer and a resin insulating layer electrically insulating both the conductor circuit layers, characterized in that said lower conductor circuit layer is formed by etching a substrate provided with a conductor layer to remove a conductor useless portion therefrom, and a resin is filled in a concave portion formed by removing the conductor useless portion so that the surface of the resin is saue as the surface of the lower conductor circuit layer, and said resin insulating layer is comprised of a composite layer consisting of an insulating layer made from a heat-resistant resin hardly soluble in acid or an oxidizing agent as a lower layer and an adhesive layer for electroless plating made from a heat-resistant resin as an upper layer, and said upper conductor circuit layer is formed on said adhesive layer constituting the upper layer of said resin insulating layer. PA0 (14) In the invention described in the item (13), it is preferable that the resin filled in the concave portion formed by removing the conductor useless portion is a cured product of non-solvent resin. PA0 (15) In the invention described in the item (13), it is preferable that the resin filled in the concave portion formed by removing the conductor useless portion contains inorganic particles. PA0 (16) It is desirable that cured particles of a heat-resistant resin soluble in the acid or oxidizing agent and having an average particle size of 0.1-2.0 .mu.m are dispersed in the insulating layer of the item (13). PA0 (17) A method of producing a multilayer printed circuit board comprising an upper conductor circuit layer, a lower conductor circuit layer and a resin insulating layer electrically insulating both the conductor circuit layers, which comprises the following steps (a)-(e): PA0 (18) In the invention described in the item (17), it is preferable that said resin insulating layer formed in the step (d) is a composite layer consisting of an insulating layer made from a heat-resistant resin hardly soluble in acid or an oxidizing agent as a lower layer and an adhesive layer for electroless plating made from a heat-resistant resin as an upper layer. PA0 (19) It is desirable that cured particles of a heat-resistant resin soluble in the acid or oxidizing agent and having an average particle size of 0.1-2.0 .mu.m are dispersed in the insulating layer of the item (18). PA0 (20) A resin filler of non-solvent filled in a concave portion created on a surface of a wiring substrate or in a through-hole formed in the substrate, in which bisphenol type epoxy resin is used as a resin component and an imidazole curing agent is a curing agent. PA0 (21) A resin filler of non-solvent filled in a concave portion created on a surface of a wiring substrate or in a through-hole formed in the substrate, in which bisphenol type epoxy resin is used as a resin component and an imidazole curing agent is a curing agent and inorganic particles are used as an additive component. PA0 (22) In the resin filler described in the item (20) or (21), it is preferable that the bisphenol type epoxy resin is bisphenol F-type epoxy resin. PA0 (23) In the resin filler described in the item (20) or (21), it is favorable that a viscosity of the resin filler is 0.3.times.10.sup.5 -1.0.times.10.sup.5 cps (30-100 Pa.multidot.s), preferably 0.45.times.10.sup.2 -0.65.times.10.sup.2 Pa.multidot.s at a temperature of 23.+-.1.degree. C. PA0 (24) In the resin filler described in the item (20) or (21), it is favorable that the inorganic particles have an average particle size of 0.1-5.0 .mu.m. PA0 (25) A build-up multilayer printed circuit board in which an interlaminar insulating layer and a conductor layer are alternately laminated on a surface of a wiring substrate having a conductor circuit cr a through-hole and the conductor layers are electrically connected to each other through a viahole formed in the interlaminar insulating layer, characterized in that a resin filler of non-solvent comprising a bisphenol type epoxy resin as a resin component and an imidazole curing agent as a curing agent is filled in a concave portion created on the surface of the wiring substrate or in the through-hole formed in the substrate. PA0 (26) A build-up multilayer printed circuit board in which an interlaminar insulating layer and a conductor layer are alternately laminated on a surface of a wiring substrate having a conductor circuit or a through-hole and the conductor layers are electrically connected to each other through a viahole formed in the interlaminar insulating layer, characterized in that a resin filler of non-solvent comprising a bisphenol type epoxy resin as a resin component, an imidazole curing agent as a curing agent and ceramic filler is filled in a concave portion created on the surface of the wiring substrate or in the through-hole formed in the substrate. PA0 (27) A build-up multilayer printed circuit board in the item (25) or (26) a roughend layer is formed on a conductor face of an inner wall of the through-hole.
On the other hand, the resin insulating layer being inevitable in the above additive process is generally formed by simply applying one kind of the resin composition. However, as the printed circuit board is recently used in various fields, it is very difficult to form the resin insulating layer so as to exhibit excellent application adaptability in all fields. Lately, the resin insulating layer constituting the printed circuit board tends to become thinner in accordance with the high performance and high densification of electric parts, which is difficult to ensure the insulating property between the conductor layer and another conductor layer and hence results in the lowering of reliability in the multilayer printed circuit board.
Furthermore, the surface of the resin insulating layer is roughened by using an acid or an oxidizing agent and dissolving and removing particles of heat-resistant resin soluble in the oxidizing agent or the like, which are dispersed in heat-resistant resin hardly soluble in the oxidizing agent or the like. However, if aggregation or the like is caused in the heat-resistant resin particles to be dissolved and removed, the depth of the roughened layer is not constant. Particularly, when the resin insulating layer is thin, the depth of the roughened layer may arrive at the thickness of the resin insulating layer and hence the interlaminar insulation property naturally required in the resin insulating layer considerably lowers.
It is a main object of the invention to provide a new structure of a resin insulating layer capable of providing a multilayer printed circuit board having an excellent interlaminar insulation property without lowering the peel strength even if the thickness of the resin insulating layer is thin.
It is another object of the invention to provide a multilayer printed circuit board having an excellent resolution capable of conducting exposure and development treatments under same conditions without forming unevenness on the surface even if an innermost layer of conductor circuits is formed by an etching treatment through the subtractive process as well as a method of producing the same.
It is the other object of the invention to provide a multilayer printed circuit board having excellent resolution, interlaminar insulation property and resistance to cool and heat shocks.
It is a still further object to provide a resin filler for smoothening the surface of the substrate suitable for the multilayer printed circuit board.